1. Field of the Invention
This invention generally relates to electro-optical systems for reading indicia such as coded symbols and, more particularly, to simplifying the assembly and automatically testing the components in such systems.
2. Description of the Related Art
Electro-optical systems for reading coded indicia such as bar code symbols are well established in the art. See, for example, U.S. Pat. Nos. 4,251,798; 4,387,297; and 4,409,470, as well as the many other U.S. patents issued to Symbol Technologies, Inc., the assignee of the instant application.
Thus, a typical system includes a light source, such as a laser diode, for emitting a light beam toward a symbol for reflection therefrom. The reflected light is detected by a light sensor, such as a photodiode, over a field of view. The light beam and/or the field of view is scanned by an oscillating scan drive, typically including a light reflector. The sensor generates an electrical signal indicative of the symbol being scanned. This electrical signal is thereupon processed in a well known manner, typically to identify the item bearing the symbol, and then using this identification for inventory or check-out purposes. These electrical and optical components are often mounted in a hand-held, portable head or housing.
The assembly of the various system components is labor intensive. Various components, for example, the photodiode, are soldered to a printed circuit board on which electrical circuitry, for example, the signal processing, control and drive circuitry are mounted. Typically, a flexible strip cable is soldered to the laser diode and the circuit board. Also, an electro-magnetic interference (EMI) shield is soldered to the circuit board. Many screws are used to secure various system components in place within the housing, and even the housing itself is composed of parts that are fastened together with screws.
Aside from the labor-intensive nature of assembling the components by soldering and by manually turning multiple screws, there is also the labor-intensive procedure of manually adjusting certain optical components. Optical adjustment among the light source, light sensor, light reflector, and scan drive is critical for proper system operation. At present, three adjustments are required to manually set the "alignment" of the optical sub-system. First, the power level of the laser diode is adjusted manually by the operator by holding a power meter in front of the laser beam and simultaneously manually adjusting a potentiometer on the circuit board. Second, the position or height of the laser beam is set by adjusting the position of a laser spot from the laser beam to the center of a scan mirror on the light reflector. This is accomplished by the operator manually turning an adjustment screw while watching the location of the laser spot on a video monitor that shows a zoomed-in picture of the reflector. Third, the sweep or amplitude of the scan drive is set by lowering a mirror in the path of the laser beam so as to reflect the laser beam onto a movable plate. On the plate are etched-in tolerances for the scan width (representing the amplitude). The operator manually adjusts a potentiometer on the circuit board until the scan width falls into predetermined specifications.
After these initial adjustments are performed in a test fixture, they must be manually verified after the optical sub-assembly is mounted in the housing in which they are to finally reside. These final adjustments are not guaranteed to take place (unit could be assembled without full alignment taking place) or not be fully within specifications. Sometimes, re-work is necessary to correct the alignment, thereby leading to increased manufacturing cost and complexity.